# Max Planck

Max Planck around 1930

Max Karl Ernst Ludwig Planck (born April 23, 1858 in Kiel , † October 4, 1947 in Göttingen ) was an important German physicist in the field of theoretical physics . He is considered the founder of quantum physics . For the discovery of a constant later named after him in a basic physical equation, Planck's quantum of action , he received the 1919 Nobel Prize for Physics in 1918.

After studying in Munich and Berlin, Planck first accepted a call to Kiel in 1885 , before moving to Berlin in 1889. There Planck dealt with the radiation of black bodies and in 1900 was able to present a formula - the Planck radiation formula later named after him  - which correctly described this radiation for the first time. In doing so, he laid the foundation for modern quantum physics.

## Life

### Birth and origin

Letter signed by ten-year-old Max Planck

Max Planck was born on April 23, 1858, the sixth child of Wilhelm von Planck (1817–1900) and his second wife Emma. Patzig (1821–1914) was born and, as evidenced by the handwritten entry in the church register of the St. Nikolai parish in Kiel, originally received the first name Marx . It is not clear whether this was a mistake, but Planck used the name Max throughout his life . He had four siblings (Hermann, Hildegard, Adalbert and Otto) and two half-siblings ( Hugo and Emma ) from his father's first marriage ).

Planck's father Wilhelm von Planck came from a traditional family of scholars. His great-grandfather Georg Jakob Planck was the town clerk in Nürtingen , his grandfather Gottlieb Jakob Planck (1751–1833) and his father Heinrich Ludwig Planck (1785–1831) were both professors of theology in Göttingen . Wilhelm von Planck himself was a law professor in Kiel at the time of Max Planck's birth ; previously he had taught in Basel and Greifswald . His brother Gottlieb Planck (1824-1910) was also a lawyer and taught in Göttingen, he was one of the authors of the civil code .

Planck's mother, Emma, ​​came from Greifswald , where her father was an accountant in the provincial authority. State and administrative officials as well as pastors dominated her family. Emma Planck was always ascribed a "lively temperament", even after the death of her husband she frequented the academic circles of Munich, where she was very popular. Max Planck remained closely connected to her until her death on August 4, 1914.

### 1867–1874: School time in Munich

Max Planck as a schoolboy (1874)

Max Planck spent the first years of his life in Kiel until the family moved to Munich in 1867 , where his father had received a call to the chair of civil procedural law. From May 14, 1867 , Planck, who had previously been a student of the Sexta at the Kiel School of Academics , attended the first Latin class at the Maximiliansgymnasium . The multi-talented Planck was a good, but not an outstanding student and was considered the favorite of the teachers, who attested that he had "a very clear, logical head despite all of his childhood."

Even if there was no science class at Maximiliansgymnasium, Planck first came into contact with physics here. His math teacher Hermann Müller, whom Planck described in retrospect as “a man who is in the midst of life, astute and funny”, taught the students the basics of astronomy and mechanics , which were part of the high school class in his subject. Planck remembered the principle of conservation of energy introduced by the teacher through a “drastic” and clear example as particularly formative . He has accepted this "first [] law for him, which has an absolute validity independent of human beings, [...] like a message of salvation [...]".

Planck's classmates at Maximiliansgymnasium included the later founder of the Deutsches Museum , Oskar Miller , and Walther von Dyck , who became known as a mathematician and science manager. The children of many wealthy and respected families also attended school, including the son of the writer Paul Heyse and Planck's future brother-in-law Karl Merck, son of the banker Heinrich Johann Merck .

In the summer of 1874, at the age of 16, Planck passed the Abitur as fourth-best of his class. The upcoming choice of subject was not easy for him, at first he vacillated between natural sciences, classical philology and studying music. Planck, who had perfect pitch, played the piano and cello and regularly accompanied church services on the organ . He was also an excellent singer and was a boy soprano member of the school and church choir. In addition, conducted and composed he songs for little plays and house music, which at that time for the educated middle class was a common pastime. As a student, he later even composed an operetta entitled Die Liebe im Walde , which, however, has not survived.

When looking for a subject, Planck initially considered studying music, but saw no career prospects in it and decided on physics . The Munich physics professor Philipp von Jolly , from whom Planck asked about the prospects in 1874, commented on Planck's interest in physics with the remark that "almost everything in this science has already been researched and only a few insignificant gaps have to be closed." - a view held by many physicists at the time.

### 1874–1879: Studies in Munich and Berlin

Planck as a student in Berlin (1878)

In the winter semester of 1874 , Planck enrolled at the Ludwig Maximilians University in Munich to study mathematics and natural sciences. There Philipp von Jolly , who, according to his contemporaries, was “an admirable lecturer with unsurpassable clarity and elegance of presentation”, became his academic teacher. Planck attended further physics lectures with Wilhelm Beetz , his mathematics teachers were Philipp Ludwig von Seidel and Gustav Bauer , whose mathematical college "internally satisfied and stimulated" him.

With von Jolly, who at that time tried with little success to experimentally determine the acceleration due to gravity , Planck got to know the difficulties of physical research. During this time, Planck undertook the only independent experiments of his entire scientific career when he was investigating whether the "semi-permeable walls" assumed by theoretical physicists actually existed. To this end, he dealt with the diffusion of hydrogen through heated platinum , which in this constellation is actually semi-permeable. This knowledge was later taken up for experiments in physics and chemistry.

At the Academic Choral Society AGV Munich , to which he, like his brothers, belonged, Planck met Carl Runge (1856–1927), two years his senior , who also studied mathematics and physics and subsequently became known as a mathematician. In the spring of 1877, Planck went on a hike to Italy with two friends , which Runge later joined. Planck's biographers rate this trip, during which there were many, mostly philosophical, discussions as an important event in Planck's later youth. Especially Runge, “who had the courage to venture into bold thought excursions, […] [startled] his fellow student Planck with the then rebellious new question whether it could not be that the Christian Church could do more harm than good for people and for them World. ”( Fischer : Der Physiker) For Planck, who came from a traditional family, these were completely new ideas.

In the winter semester of 1877, Planck and Runge moved to Berlin for a year , where he studied at the Friedrich-Wilhelms-University with the famous physicists Gustav Kirchhoff and Hermann von Helmholtz , whom he had already met in Munich. However, Planck was soon disappointed by the lectures of the scientists he admired and, looking back, wrote: “[Helmholtz] was never properly prepared, he always spoke hesitantly, [...] moreover, he kept miscalculating [...] and we had the feeling that he was at least as bored with this lecture as we were. ”Kirchhoff, on the other hand, gave extensively prepared and formulated lectures, but Planck found them to be“ dry and monotonous. ”That is why Planck, who also heard the mathematician Karl Weierstrass in Berlin , mainly formed his mind in self-study from the writings of Rudolf Clausius , who had dealt with heat theory, which subsequently also became Planck's field of work. Clausius had formulated the first two main principles of thermodynamics for the first time , whereby Planck already knew the first from his school days as the “principle of the conservation of energy”. Planck chose the second main clause as the subject of his dissertation.

In October 1878, Planck, now back in Munich, passed the “ state examination for teaching at secondary schools” in the subjects of mathematics and physics. At that time, this was the aim of most physics students' study goal, as only the profession of teacher promised a regular job. In contrast, following the tradition of his family, Planck decided to pursue a university career and only worked as a substitute for a short time at his former school at the end of 1878. On February 12, 1879, he submitted his dissertation on the second law of mechanical heat theory , in which, according to the reviewers, he “achieved far more than is commonly required by an inaugural dissertation.” His independent work on the was also particularly emphasized chosen topic as well as its expertise. Planck also passed the oral exam on May 30th of that year with flying colors. The commission, consisting of von Jolly (physics), Bauer (mathematics) and Adolf von Baeyer (chemistry), awarded him the grade I with the distinction summa cum laude . The written examination in various areas of physics, which was necessary for a doctorate at the time, did not cause Planck any difficulties either, so that he received his doctorate on June 28, 1879 after a public lecture on the development of the concept of heat and a subsequent general discussion.

### 1880–1885: private lecturer in Munich

As early as 1880, Planck submitted his habilitation thesis on the equilibrium of isotropic bodies at different temperatures , in which he used the general knowledge from his dissertation to solve various physicochemical problems. After a public trial lecture on the principles of mechanical gas theory with subsequent discussion, Planck received his habilitation on June 14, 1880. At the age of just 22 he was a university professor and was appointed to the Munich University as a private lecturer.

There he held his first lecture on analytical mechanics from the winter semester of 1880 - unpaid and still living with his parents - and in the following years expanded this course into a cycle that dealt with all the important areas of physics from a theoretical point of view. At the same time, he tried to make a name for himself as a scientist in order to soon be offered a professorship. In 1883 he received one from the Aschaffenburg Forest Academy , but declined the appointment after consulting Helmholtz because he saw no scientific perspective in it. Planck, who at that time was already engaged to Marie Merck, felt an increasing "urge for independence" and was dissatisfied with his situation, especially the dependence on his father's maintenance.

During this time, Planck received little attention from experts, and neither his dissertation nor his habilitation thesis received any attention. Nonetheless, Planck continued his research in the field of heat theory and devoted himself to entropy during his time in Munich . To do this, he examined changes in the state of aggregation , gas mixtures and solutions .

### 1885–1889: Professorship in Kiel, marriage to Marie Merck

Main building of the University of Kiel (1893)

In April 1885, the Christian-Albrechts-Universität zu Kiel appointed Planck as associate professor for theoretical physics. The rather small university had already tried to set up such a position since 1883 and initially hired Heinrich Hertz as a private lecturer on Berlin's recommendation . However, since the establishment of the professorship was delayed, he accepted a call from the TH Karlsruhe in 1884 . During the subsequent search by the Kiel faculty for a successor, the choice quickly fell on Planck, as he “had the longest and most successful activity among the youngest docents in theoretical physics”.

After brief negotiations, during which he benefited from his father's good relations in Kiel, Planck was appointed professor in Kiel on May 2, 1885. Although there were only a few students in his field in Kiel, he was able to confirm and expand his reputation as a physicist here. Planck, who now had an annual salary of 2,000 marks plus housing allowance and college grants for the students, was now financially on his own two feet and, after becoming engaged to her in the summer of 1886, was able to meet his long-term girlfriend Marie Merck (1861 –1909) marry. Their first son Karl (1888–1916) was born on March 9, 1888, followed by twin daughters Emma (1889–1919) and Grete (1889–1917) in April 1889, and the second son Erwin (1893–1945) in 1893. , who later became State Secretary in the Reich Chancellery and was one of the murdered assassins on July 20, 1944 .

During his time in Kiel, Planck took part in an 1884 competition organized by the Philosophical Faculty of the University of Göttingen for the year 1887 “On the nature of energy”. For his monograph The Principle of Conservation of Energy he was awarded the second prize, and since the first prize was not awarded, Planck unofficially emerged as the winner of the competition. The jury particularly emphasized "the methodical way of thinking, the thorough mathematical and physical education of the author [and] the prudence of his judgment". Presumably he was denied the first prize, because in his treatise he preferred the work of Helmholtz to that of the Göttingen professor Wilhelm Eduard Weber . At that time there was a heated scientific argument between the two physicists.

In Kiel, Planck finally decided to focus on theoretical physics as a specialist area, which was initially an unusual decision for the time. In Germany there were only two chairs for this branch of physics, which the dominant experimental physicists saw as a necessary evil or merely as an auxiliary science for their research.

### From 1889: Professorship in Berlin

Main building of the Berlin University (around 1900)

In April 1889, Planck was appointed to the Friedrich Wilhelms University in Berlin. There he succeeded Gustav Kirchhoff , who died unexpectedly in October 1887 . Originally, the Philosophical Faculty, to which the Chair of Physics belonged at the time, tried to win the 44-year-old Ludwig Boltzmann from Graz . Boltzmann was one of the leading theoretical physicists of the time and thus corresponded to the requirement profile of the faculty, which was looking for "authorities in strong manhood". When this plan failed, the appointment committee proposed Heinrich Hertz and Planck as possible candidates in November 1888 . Since Hertz did not want to leave his position in Karlsruhe, Planck finally received the call. Initially, Planck was only an associate professor - the university was unsure whether the young physicist met the high requirements - but was appointed full professor in 1892 and now held the chair of theoretical physics.

Immediately after taking office, Planck joined the German Physical Society in Berlin , where he was soon also active as treasurer. In 1899, Planck played a key role in the transformation and renaming of the company into the German Physical Society . As early as 1894, Planck was elected to the Royal Prussian Academy of Sciences in Berlin at the suggestion of Helmholtz . At just 35 - the average age of the Academy was over 60 - Planck was a member of one of the most renowned scientific societies in Europe. This was another important step in Planck's career.

Memorial plaque on the house at Wangenheimstrasse 21, where Planck lived from 1905 to 1944

In Berlin, Planck was not only scientifically but also socially more involved than in Kiel. In the villa colony of Grunewald , where many Berlin professors lived, Planck also had a house built and in 1905 moved with his family, to which Erwin (1893–1945), who was born in 1893, belonged, to Wangenheimstrasse 21. The historian lived in the neighborhood Hans Delbrück , the theologian Adolf von Harnack and the physician Karl Bonhoeffer , with whose families the Plancks were friends. Planck soon became close friends with Joseph Joachim (1831–1907), the director of the Academic University of Music , with whom he often played music. At this time, Planck also dealt with music theory problems, especially the tonal differences between natural and tempered tuning .

Planck held his lectures in a six-semester cycle, dealing with mechanics , electromagnetism , optics , thermodynamics and finally special problems from theoretical physics in three years each . He did not use a manuscript in his lectures, only occasionally he made sure with his notes that his calculations and derivations were correct. He developed all topics and relationships from simple formulas and equations, thus allowing his listeners to understand the relationships between the respective subject areas. Planck was highly valued by his students, as he spoke clearly and fluently and his lectures were easy to understand. Due to his clear, sober formulations, many initially felt him to be impersonal and cautious, especially since he did not mention his own authoritative contributions to quantum theory, but presented them as well as all other topics. Lise Meitner , who had previously studied with Boltzmann, who is known as a rousing speaker in Vienna, said in retrospect that she “learned very quickly how little my first impression had to do with Planck's true personality. [...] He was of a rare purity of mind and inward straightforwardness, which corresponded to his outer simplicity and simplicity. "

#### Radiation law and quantum theory, relativity theory

From the middle of the 1890s, Planck dealt with radiation equilibria and the theory of thermal radiation and tried to derive the laws of radiation from thermodynamic considerations. On December 14, 1900, he presented the Physical Society with an equation that correctly described black body radiation . The equations found up to that point, the Viennese radiation law and the Rayleigh-Jeans law , were only able to reproduce part of the radiation spectrum without deviations. In the course of working on his radiation law, Planck gave up his reservations about an atomistic-probabilistic view of entropy. At the same time he laid the foundation for quantum physics when he only allowed certain discrete energy states for the oscillators , which were responsible for the radiation in his model concept . As part of this work, Planck also introduced Planck's quantum of action , a fundamental natural constant , into physics.

→ For a detailed description, see the section Planck's law of radiation and quantum of action .

In 1905, Planck read the treatise On the Electrodynamics of Moving Bodies by Albert Einstein, who was still unknown at the time, and in the following years devoted himself intensively to the special theory of relativity introduced therein . Planck was instrumental in ensuring that Einstein's work received the necessary attention. As early as March 1906, he gave a lecture to the Physical Society in Berlin and was in correspondence with Einstein, who at that time was still living in Bern. Planck defended the new concept against critics and tried successfully to refute the experiments of Walter Kaufmann from Göttingen , whose measurements apparently contradicted the theory. As early as September 1908, when the mathematician Hermann Minkowski at the meeting of the German Natural Scientists and Physicians in Cologne the time introduced as the fourth dimension, the special theory of relativity had prevailed among experts. Regardless of his promotion of Einstein's theory of relativity, Planck rejected his interpretation of the radiation problem, the so-called light quantum hypothesis .

#### Marie Planck dies, marries Marga von Hoeßlin

On October 17, 1909, Marie Planck died after a long illness, probably from tuberculosis or bronchial carcinoma . For Planck, who had been happily married to Marie for 23 years, her death was “a terrible blow”. He also wrote to Wilhelm Wien : "[...] I hope that with the tasks that I have been given by caring for the children and by science, my strength will also come back."

On March 14, 1911, Planck married a niece of his late wife, Margarete (Marga) von Hoeßlin (1882–1949). On December 24, 1911, Hermann Planck († 1954) was born as their first child. The marriage to Marga, 25 years his junior, was not approved by all colleagues, but the 53-year-old Planck soon regained his strength through the new relationship and also resumed regular music-making in his house. The regular guests included the physicists Wilhelm Westphal , Eduard Grüneisen , Otto von Baeyer and Otto Hahn as well as the Delbrück and Harnack families . Audiences and guests at other events in the Freundeskreis were Robert Pohl and Gustav Hertz and Lise Meitner , whom Planck remembered as exuberant and carefree during this time.

#### Solvay Conference

The participants of the first Solvay conference. Planck (back row, 2nd from left) stands in front of the board on which his radiation law can be read.

In October 1911, Planck took part in the first Solvay conference initiated by his colleague Walther Nernst , at which the consequences of his radiation law for physics were to be discussed. The conference itself was unsuccessful - Albert Einstein later described it as “like a lamentation for the ruins of Jerusalem” - but heightened the physicists' awareness of the problems raised and led to an increasing number of young physicists grappling with quantum theory. This generation finally developed modern quantum mechanics in the 1920s .

Planck himself was extremely skeptical about further developments and continued to try to bring his law of radiation into harmony with classical physics. To this end, he presented the so-called “second” and “third quantum theory” in the following years, which, however, were unsuccessful due to the rapid development of quantum physics. However, this work formed an important basis for further research. Among other things, Planck pointed out the fact that atomic vibrations must still exist even at absolute zero .

#### student

Although Planck read as a professor in Berlin for 37 years and was valued as a teacher by his students, he did not establish his own school because he had only a few doctoral students and rarely came into contact with them. A scientific "enterprise" therefore did not arise at his institute.

Many of Planck's twenty or so doctoral students later became outstanding scientists themselves:

### From 1912: Permanent secretary of the Prussian Academy of Sciences

On March 23, 1912, Max Planck was elected "permanent secretary" of the Prussian Academy of Sciences , founded in 1700 . Together with three other permanent secretaries he formed the executive committee of the academy, each of whom took turns chairing the entire academy for four months. Planck now held an influential office and increasingly became the “central figure of contemporary physics” ( Dieter Hoffmann : Max Planck: The emergence of modern physics), as had been the case before Hermann von Helmholtz , who died in 1894 . For Planck, the focus was not only on its own research, but also on the development of all of physics and science in general.

Since about his election as permanent secretary of the Prussian Academy, Planck tried to bring Albert Einstein to Berlin, who however preferred to stay in Switzerland and rejected the monarchy . In the early summer of 1913, Planck traveled to Zurich with Walther Nernst and made Einstein an offer to become an academy member and professor without teaching duties at his own new institute at Berlin University. Einstein accepted in December and took up his new position on April 1, 1914. Planck was also rector of the Friedrich-Wilhelms-Universität during the academic year 1913/1914.

#### First World War

When Germany became a party in the First World War with the mobilization and declaration of war on Russia on August 1 and against France on August 3, 1914 (see July crisis ), Planck welcomed this step and was grateful to experience this “wonderful time”. Politically, he was conservative and loyal to the state, he was also patriotic and loyal to the emperor. Like most of his colleagues, he shared the enthusiasm of the population and used the university's annual foundation festival on August 3rd to give his physical lecture a patriotic appeal. The war is "about goods and blood, about honor and perhaps about the existence of the fatherland". Planck was also one of the signatories of An die Kulturwelt !, which became known as the Manifesto of 93 . In it, well-known scholars contradicted the reports of German war crimes in neutral Belgium, which were described as hostile propaganda , and justified German militarism .

When criticism arose, Planck initially asserted that he had allowed himself to be won over to a signature in favor of the manifesto without even having read it. But only about two weeks later, Planck also signed the declaration of the university lecturers of the German Reich , according to which "Our belief is that the whole of Europe depends on the victory that German" militarism "will fight for". Nevertheless, after 1945 some authors asserted that Planck later "distanced himself" from his signature on the manifesto. In fact, Planck had defended the manifesto in an open letter to his Dutch colleague Hendrik Antoon Lorentz in 1916 on the grounds that it was “an express declaration that German scholars and artists do not want to separate their cause from that of the German army. Because the German army is nothing else than the German people in arms, and like all professions, the scholars and artists are inextricably linked with it ”. In a personal letter to Lorentz, Planck also explained that his open letter was “a kind of revocation, but only with regard to the wording, not the meaning” of both texts.

In 1916, Planck also prevented members from “enemy countries” from being expelled from the Academy. Some authors interpret this as a sign that Planck did not share the chauvinism that was widespread among academics at the time . However, he justified his advocacy differently: International cooperation in science could be reconciled with “ardent love and active work for one's own fatherland”.

Planck's sons Karl and Erwin were both employed as soldiers, his daughters Emma and Grete as nurses in the First World War. Erwin Planck, the younger of the two brothers, was taken prisoner by the French on September 7, 1914 after an injury . Karl Planck died on May 16, 1916 near Verdun . Planck didn’t show anything and continued to dutifully go about his work, many around him did not find out about his son's death until weeks later.

On May 15, 1917, Planck's daughter Grete (* 1889) died in the puerperium of a pulmonary embolism , just a few days after the birth of her first child . Her twin sister Emma took care of the daughter.

In January 1919 Emma Planck married Grete's widower, Professor Ferdinand Fehling from Heidelberg . On November 21, 1919, she also died giving birth to her first child, and the daughter again survived.

### Weimar Republic

In the turmoil of the post-war period, Planck, meanwhile the highest authority in German physics, gave his colleagues the slogan "persevere and keep working". This also meant avoiding political statements, which could certainly lead to political consequences: When Einstein's theory of relativity was discredited by the public with increasing aggressiveness around 1920 and Einstein was also attacked personally, Planck refused, despite a recommendation from the Prussian Ministry of Education and Culture To have a statement from the Academy of Sciences issued in favor of Einstein. Instead, Heinrich Rubens and Walther Nernst published a defense of Einstein and his theory under their own name.

In October 1920 Fritz Haber and Max Planck founded the Notgemeinschaft der deutschen Wissenschaft , which specifically took on the promotion of research in need; a large part of the funds came from abroad. He also held leading positions in the Berlin University, the Prussian Academy of Sciences, the German Physical Society and was Senator of the Kaiser Wilhelm Society (KWG; later the Max Planck Society ) from April 1916 . From 1921 to 1922 he was chairman of the Society of German Natural Scientists and Doctors .

From 1920, Planck was a church elder in the parish council of the Evangelical Grunewald parish .

Planck became a member of the DVP , Stresemann's party , which pursued liberal domestic political goals and more revisionist foreign policy. He rejected universal suffrage and later attributed the Nazi dictatorship to the "rise of rule by the masses".

### National Socialism and World War II

When the Nazis came to power in 1933, Planck was 74 years old. He was also loyal to this regime. As President of the Kaiser Wilhelm Society (KWG), he therefore wrote to Interior Minister Wilhelm Frick on July 14, 1933 , in which he stated that the Society was willing “to systematically serve the Reich with regard to race hygiene research put". Jewish friends and colleagues of Planck were humiliated and, above all, forced out of their offices by the Law on Civil Servants, hundreds of scientists left Germany. Otto Hahn therefore asked Planck whether a number of recognized German professors could not be brought together for a joint appeal against this treatment of Jewish professors, to which Planck replied: "If you bring together 30 such gentlemen today, there will be 150 tomorrow who speak against it because they want to have the jobs of others. ” Fritz Haber was one of the few for whom Planck openly used his influence by trying to intervene directly with Hitler . That failed, Haber died in exile in 1934. A year later, in his role as President of the KWG (since 1930), Planck organized a memorial service for Haber. Incidentally, however, Planck continued to try to "persevere and continue working" and only asked physicists willing to emigrate in private not to leave, which he was partially successful with, and also made it possible for a number of Jewish scientists to continue working for a limited time at institutes of the Kaiser Wilhelm Society to work. Planck's presidency of the KWG ended in 1936 and, at the urging of the Nazis, decided not to run for re-election.

The political climate intensified further and was now directed against Planck. Johannes Stark , representative of " German Physics " and President of the Physikalisch-Technische Reichsanstalt, insulted Planck, Sommerfeld and Heisenberg as "white Jews" in an SS magazine and polemicized against the entire theoretical physics. The “Hauptamt Wissenschaft” examined Planck's origin, but only came up with the result that he was “one sixteenth Jewish”.

In 1938 Planck celebrated its eightieth birthday: during the official ceremony of the DPG, the French physicist Louis de Broglie was awarded the Max Planck Medal in the run-up to a new war. Planck received around 900 congratulations, all of which he answered personally and individually.

In late 1938, the Academy has been brought into line , Planck resigned in protest. Despite his old age, he still went on numerous lecture tours, for example to the Baltic States in 1937 with the famous lecture Religion and Natural Science, and in 1943 he climbed several three-thousand-meter peaks while on vacation in the Alps.

During the Second World War , Planck had to leave Berlin due to the air war. On March 1, 1943, he found quarters with the industrialist Carl Still , whose manor house still stands on the former site of Rogätz Castle . In 1942 he wrote: "I have grown up with the burning desire to get through the crisis and live until I can witness the turning point, the beginning of an ascent." With "Ascent", however, he is less likely to see a military victory for the National Socialist regime rather than meant a political and moral new beginning after its end. Because Planck was well aware at the time of the crimes committed by Germans, provided that he could not see them with his own eyes. In May 1943 he said to Lise Meitner : "Terrible things have to happen, we have done terrible things." At the end of October 1943 he was supposed to give a lecture in Kassel; therefore he stayed with relatives from October 22nd to October 23rd when Kassel was the target of a devastating air raid. He saw his relatives being bombed out. In February 1944, his house in Berlin was completely destroyed by an air raid.

On July 23, 1944, his son Erwin Planck was arrested for participating in the uprising of July 20, 1944 and taken to the Gestapo headquarters . With several petitions, Father Planck asserted his son's innocence in accordance with the indictment. He wrote to Himmler : "Due to the close relationship that binds me to my son, I am sure that he has nothing to do with the events of July 20th." When Erwin Planck of the People's Court on October 23rd, 1944, nevertheless Having been sentenced to death, Father Planck wrote to Hitler : “My Führer! I am deeply shocked by the news that my son Erwin has been sentenced to death by the People's Court. The recognition of my achievements in the service of our fatherland, which you, my Fuehrer, have repeatedly expressed in the most honorable manner, entitles me to the confidence that you will listen to the request of those who are eighty-seven years old. As a thank you from the German people for my life's work, which has become an immortal intellectual property of Germany, I ask for the life of my son. ”Planck sent further petitions to Hermann Göring and again to Himmler, who promised to convert the death penalty into a prison sentence should have. Nevertheless, Erwin Planck was executed in Plötzensee on January 23, 1945 .

When the area around Rogätz also became a combat zone, the Planck couple fled to the neighboring forest. It spent a few days in the open air with hundreds of others, then it was taken into the hut of a family of milkers. The area came between the fronts of the advancing Western and Soviet allies. An American officer evacuated the couple to the undestroyed Göttingen, where they stayed with a niece of Planck's.

### Late years

The grave of Max Planck and his family in the Göttingen city cemetery

After the end of the war, the Kaiser Wilhelm Society was rebuilt from Göttingen under the leadership of Ernst Telschow , and Max Planck became its acting president. After his return from internment in England, Otto Hahn succeeded him on April 1, 1946. Since the British occupying power insisted on a different name, the association was renamed the Max Planck Society on September 11, 1946 at the Clemens Hofbauer College in Bad Driburg . Max Planck was appointed honorary president.

Despite increasing health problems, Planck went on lecture tours again. In July 1946 he was the only German invited to take part in the celebrations of the Royal Society for Isaac Newton's 300th birthday . On October 4, 1947, Max Planck died of the consequences of a fall and several strokes. His grave is in the Göttingen city cemetery , where a number of other Nobel Prize winners are buried.

### Religion and science

In the last decades of his life, Planck turned to the philosophical borderline questions of his physical worldview. He was influenced philosophically by Immanuel Kant and theologically by Adolf Harnack . In lectures and essays he took the view that religion starts from a belief in God and encompasses the area of ethics , that natural science strives towards God as a scientific-empirical knowledge, but that it can only end with a "scientific power". Planck affirmed the believed reality of God. In addition, there was his criticism of a pseudo-metaphysics that tried to derive impermissible proofs of God from quantum theory . In addition, Planck criticized the absolute establishment of “religious symbols” by the churches, that is, mythological statements. He was a member of the Evangelical Church until his death.

## plant

### entropy

The thermodynamics , which was referred to the late 19th century as a "mechanical theory of heat" was created at the beginning of this century of trying out, to understand the operation of steam engines and improve their efficiency. In the 1840s, several researchers independently discovered and formulated the law of conservation of energy , which is now also known as the first law of thermodynamics . In 1850 Rudolf Clausius formulated the so-called second law, which says that a voluntary (or spontaneous) transfer of energy is only possible from a warmer to a colder body, but not the other way around. In England at this time William Thomson came to the same conclusion.

Clausius continued to generalize his formulation and came up with a new formulation in 1865. To this end, he introduced the term entropy , which he defined as a measure of the reversible supply of heat in relation to the absolute temperature: ${\ displaystyle S}$

${\ displaystyle \ mathrm {d} S = {\ frac {\ mathrm {d} Q} {T}}}$

The new formulation of the second main clause, which is still valid today, was: "Entropy can be generated, but never destroyed". Clausius, whose work Planck read as a young student during his stay in Berlin, successfully applied this new law of nature to mechanical, thermoelectric and chemical processes as well as changes in the state of matter .

In his dissertation in 1879, Planck summarized Clausius' writings, pointing out contradictions and inaccuracies in their formulation in order to clarify them afterwards. In addition, he generalized the validity of the second law to all processes in nature, Clausius had limited its application to reversible processes and thermal processes. Planck also dealt intensively with the new concept of entropy and pointed out that entropy is not only a property of a physical system, but also a measure of the irreversibility of a process: If entropy is generated in a process, it is irreversible, since entropy is in accordance with it the second law cannot be destroyed. In the case of reversible processes, the entropy therefore remains constant. He presented these facts in detail in 1887 in a series of treatises entitled On the Principle of Increasing Entropy . Planck's work received little attention at that time, and many physicists regarded entropy as a "mathematical ghost".

In his study of the concept of entropy, Planck did not follow the prevailing molecular, probabilistic interpretation of the time, as this did not allow absolute proof of general validity. Instead, he took a phenomenological approach and was also skeptical of atomism. Even if he later gave up this position in the course of his work on the law of radiation, his early work shows "impressively the great power of phenomenological thermodynamics in solving specific physicochemical problems [...]" ( Dieter Hoffmann : Max Planck: The emergence of modern physics).

Planck's understanding of entropy included the knowledge that the maximum of entropy corresponds to the state of equilibrium. The associated conclusion that all laws of thermodynamic equilibrium states can be derived from the knowledge of entropy corresponds to the modern understanding of such states. Planck therefore chose equilibrium processes as his research focus and, based on his habilitation thesis, researched, for example, the coexistence of states of aggregation and the equilibrium of gas reactions. This work on the border to chemical thermodynamics also received a great deal of attention from the rapidly expanding chemical industry at that time.

Independently of Planck, the American Josiah Willard Gibbs had also discovered almost all of the knowledge that Planck gained about the properties of physical-chemical equilibria and published them from 1876 onwards. These essays were unknown to Planck, they did not appear in German until 1892. However, both scientists approached the topic in different ways, while Planck was concerned with irreversible processes, Gibbs looked at the equilibria. This approach was ultimately able to prevail because of its simplicity, but Planck's approach is considered to be “more general”.

### Electrolytes and solutions

In addition to his research on entropy, Planck also dealt with electrical processes in solutions in the first decade of his scientific work . Among other things, he succeeded in theoretically deriving the dependence on conductivity and dilution of a solution, thus establishing the modern electrolyte theory . He was also able to theoretically derive the conditions for the freezing and boiling point changes in dilute solutions that Raoult and van 't Hoff had found in 1886.

### Planck's law of radiation and quantum of action

After he had largely completed his work on thermodynamic equilibria and then learned that the American Josiah Willard Gibbs had previously come to the same results, Planck turned to radiation equilibria and the theory of thermal radiation in the mid-1890s. At that time little was known about the laws by which heated bodies emit rays of heat and light. In 1859 Gustav Kirchhoff postulated the central importance of a universal radiation function, which is only dependent on frequency and temperature, for describing thermal radiation. He introduced the concept of the black body , which completely absorbs all incident radiation. Conversely, such a black body only emits the radiation it emits. This simplifies the search for the radiation function, since the problem can be reduced to examining the radiation from a black body. ${\ displaystyle f (\ nu, T)}$

However, the experimental and theoretical hurdles were great, until 1879 was Josef Stefan the relationship between energy density and temperature as determined. Ludwig Boltzmann was able to find a law for the total radiation of a black body from this in 1884, while Wilhelm Wien of the Physikalisch-Technische Reichsanstalt in Berlin determined the so-called Vienna displacement law in 1893 . Three years later, Wien's radiation law followed , which initially confirmed the experimental results - taking into account the large measurement errors common at the time. ${\ displaystyle I}$${\ displaystyle T}$${\ displaystyle I \ sim T ^ {4}}$ ${\ displaystyle \ lambda T = constant}$

While the scientists at the Physikalisch-Technische Reichsanstalt tried to create a black body for the first time in order to be able to carry out measurements on it, Planck approached the problem from a theoretical point of view. In 1894 he tried to derive the laws of radiation physics from thermodynamic considerations. This work was the immediate continuation of his earlier research on thermodynamic equilibria and entropy, which he wanted to link in this way with the electromagnetic theory of light . This would have made it possible to interpret thermal radiation as an electromagnetic process, which from the perspective of that time would have represented a further completion of physics.

For his theory, Planck used the harmonic oscillators introduced by Heinrich Hertz in 1889 as the " Hertzian oscillator " , with which the emission and absorption of electromagnetic waves could be described. Planck transferred this concept to heat-radiating bodies and presented its results to the Prussian Academy of Sciences in March 1895 and February 1896 . In the following years he expanded this approach and published between 1897 and 1899 five treatises on irreversible radiation processes. He also succeeded in deriving Wien's radiation law from considering the radiation behavior of a cavity. When he presented these results to the Academy in May 1899, he had also come to the conclusion that this law, like the second law of thermodynamics, was universally valid. At the same time led Planck later than Planck's constant called natural constant , but did not recognize their full significance. ${\ displaystyle h}$

Comparison of the laws of Rayleigh-Jeans (red), Planck (green) and Vienna (blue) for frequencies from around 20  MHz to around 2  GHz

Measurements by Heinrich Rubens and Ferdinand Kurlbaum in the summer of 1900 showed that the deviations in Vienna's radiation law in low frequency ranges, which had been interpreted as measurement errors, were in fact serious errors in the equation itself. Rubens, who was friends with Planck, reported the results to him in October of that year and pointed out to him that for long wavelengths it was not Vienna's radiation law that had to apply , but rather the Rayleigh-Jeans law that had just been found . This in turn deviated significantly in high frequency ranges, where Wien's law provided precise values. Immediately after this conversation, Planck found a “luckily guessed interpolation formula” for the measurement results, which Rubens was able to confirm during measurements in the following days. The Planck's radiation law joined the Wien with the Rayleigh-Jeans law, both of which can be considered as borderline cases.

The preliminary result, which Planck presented to the academy on October 19 following a lecture by Kurlbaum, still contained two constants that were undefined at that time. In the weeks that followed, Planck finalized the law:

${\ displaystyle \ rho (\ nu, T) = {\ frac {8 \ pi \ nu ^ {2}} {c ^ {3}}} {\ frac {h \ nu} {e ^ {\ frac {h \ nu} {kT}} - 1}}}$

For this purpose, Planck used the atomistic-probability- theoretical justification of the entropy by Ludwig Boltzmann , which he had rejected up to that point , so he gave up the phenomenological approach he had consistently pursued up to that point and recognized his error. In retrospect, Planck described this step as an “act of desperation”. Analogous to Boltzmann's work on gas statistics from 1877, Planck only allowed certain energy states for the radiation oscillators. The law derived in this way contains the Boltzmann constant of the speed of light and Planck's quantum of action, three natural constants, otherwise only the variable parameters temperature and frequency are included. Due to the relationship found by Planck, the natural constants could be determined much more precisely in the following years than had been possible until then. ${\ displaystyle k,}$ ${\ displaystyle c}$

On December 14, 1900, Planck presented his results at a meeting of the Physical Society. According to Max von Laue , this day has since been the “birthday of quantum physics”, although none of the scientists present - Planck included - the meaning and scope of the formula or the Constants was aware. One saw in Planck's result first of all a formula that correctly represented the radiation conditions. It was not until Albert Einstein's light quantum hypothesis from 1905 and the subsequent critical analysis of Planck's law of radiation, which Einstein then worked out together with Paul Ehrenfest , that its incompatibility with classical physics became clear. It was not until 1908 that Planck himself described the energy states of the oscillators as " discrete ". ${\ displaystyle h}$

After the Solvay conference in 1911, where the problems raised by Planck's law of radiation were explained, Planck tried to bring the law of radiation into harmony with classical physics. To this end, he developed the "second quantum theory" until 1912, according to which only the emission of energy is quantized, but the absorption takes place continuously. In 1914 he presented a “third quantum theory” that managed completely without quanta. As before, he rejected Einstein's light quantum hypothesis.

The Copenhagen interpretation of quantum mechanics worked out by Bohr , Heisenberg and Pauli in the late 1920s was rejected by Planck, along with Schrödinger and Laue ; Einstein too had now become a conservative. The Heisenberg matrix mechanics found Planck "abhorrent", the Schrödinger equation he welcomed as a relief. He expected that wave mechanics would soon make quantum theory, his own child, superfluous. Science ignored his concerns. What he had found in his struggle with the old man at a young age was also true for himself: “A new scientific truth does not usually assert itself in the way that its opponents are convinced and declare themselves informed, but rather by the fact that the opponents gradually extinct and that the next generation is familiar with the truth from the start. ”Scientific autobiography, Leipzig 1948.

### music

At the end of the 19th century Planck dealt intensively with the problems of the pure intonation of choral singing with modulations .

## Awards and honors

Max Planck

### Posthumous honors

Many schools and universities are named after Max Planck, see list of Max Planck grammar schools .

At the Christian-Albrechts-Universität zu Kiel, Department of Theoretical Physics, there has been an exhibition on the life and work of Max Planck since 2014. The Max Planck Museum Kiel initiative was founded in February 2019

## Works (selection)

### Magazine articles

• About an improvement of Wien's spectral equation . In: Negoti. German Phys. Ges. Band 2 , 1900, p. 202-204 ( archive.org [accessed January 20, 2018]).
• On the theory of the law of energy distribution in the normal spectrum . In: Negoti. German Phys. Ges. Band 2 , 1900, p. 237–245 ( archive.org [accessed January 20, 2018]).
• About the law of energy distribution in the normal spectrum . In: Ann. Phys. tape 4 , no. 3 , 1901, pp. 553-563 , doi : 10.1002 / andp.19013090310 ( wiley.com [PDF; accessed January 20, 2018]).

## literature

### Biographies

• Hans Hartmann: Max Planck as a person and a thinker. Siegismund, Berlin 1938 (2nd revised edition by Hirzel, Leipzig 1948, 3rd revised edition by Ott, Basel 1953, unabridged new edition as paperback by Ullstein, Frankfurt 1964).
• Max Planck: Scientific autobiography: With a portrait and the funeral address given by Max von Laue. Barth, Leipzig 1948.
• Wolfgang Gerlach: The quantum theory: Max Planck, his work and its effect. University Press, Bonn 1948.
• Armin Hermann : Max Planck: with personal testimonies and photo documents. Rowohlt, Reinbek 1973, ISBN 3-499-50198-8 (8th edition 2005).
• John Lewis Heilbron: The Dilemmas of an Upright Man: Max Planck and the fortunes of German science. University of California Press, Berkeley 1986, with bibliography (expanded edition from Harvard University Press, Cambridge 2000, ISBN 0-674-00439-6 ).
• In German translation: John Lewis Heilbron: Max Planck: A life for science 1858–1947. From the American by Norma von Ragenfeld-Feldmann, Hirzel, Stuttgart 1988, ISBN 3-7776-0392-9 . (2nd corrected and supplemented edition by Hirzel, Stuttgart 2006, ISBN 3-7776-1438-6 ).
• Letter diary between Max Planck, Carl Runge, Bernhard Karsten and Adolf Leopold. Edited, introduced and commented on by Klaus Hentschel and Renate Tobies , ERS-Verlag, Berlin 1999 (second, expanded edition 2003).
• Ernst Peter Fischer : The physicist. Max Planck and the disintegration of the world. Siedler, Munich 2007, ISBN 978-3-88680-837-3 .
• Dieter Hoffmann : Max Planck. The emergence of modern physics. Verlag CH Beck , Munich 2008, ISBN 978-3-406-56242-6 .

• Dieter Hoffmann (Ed.): Max Planck and modern physics. Springer, Berlin 2010, ISBN 978-3-540-87844-5 (collection of articles, with bibliography).
• Jules Leveugle: La Relativité, Poincaré et Einstein, Planck, Hilbert: Histoire véridique de la Théorie de la Relativité. L'Harmattan, Paris 2004, ISBN 2-7475-6862-8 (paperback).
• Astrid von Pufendorf: The Plancks. A family between patriotism and resistance. Propylaea, Berlin 2006, ISBN 3-549-07277-5 .
• Heinrich Vogel: On the philosophical work of Max Planck. Akad.-Verlag, Berlin 1961.

### Speeches and memorials

• Emil Warburg , Max von Laue , Arnold Sommerfeld , Albert Einstein , Max Planck: On Max Planck's sixtieth birthday: Speeches. Karlsruhe 1918 (with memories from Max Planck).
• German Academy of Sciences: Max Planck in memory. Berlin 1958.
• Kockel, Macke, Papapetrou: Max Planck Festschrift 1958. Berlin 1959.
• Eugen Hintsches, Dieter Hoffmann: Max Planck: Lectures and exhibition on the 50th anniversary of death. Published by the Max Planck Society for the Advancement of Science, Press Office, Munich 1997.

### Bibliography

• Max Planck: Physical treatises and lectures: On the occasion of his 100th birthday (April 23, 1958). Published by the Association of German Physical Societies and the Max Planck Society for the Advancement of Science, Vieweg, Braunschweig 1958 (3 volumes).
• Petra Hauke: Literature about Max Planck: Inventory. Max Planck Society for the Advancement of Science, Berlin 2001, ISBN 3-927579-14-9 (publications from the archive on the history of the Max Planck Society 14).

## media

Documentaries

• Max Planck: upheaval with melancholy. Documentation, Germany 2008, 45 min., Script and direction: Jürgen Miermeister, first broadcast: April 9, 2008, summary by 3sat, review in the FAZ, online on Youtube.
• nano extra: Max Planck - The grainy world. Documentation, Germany, 2008, 30 min., Written and directed: Malte Linde, first broadcast: April 9, 2008, table of contents.

Max Planck in picture and sound

Lecture on DVD

Commons : Max Planck  - collection of images, videos and audio files
Wikisource: Max Planck  - Sources and full texts

## Individual evidence

1. ^ The Nobel Prize in Physics 1918. At: nobelprize.org.
2. ^ Heilbron: The Dilemmas of an Upright Man. Berkeley 1986, p. 1
3. Christoph Seidler: Name Surprise: Permit, Marx Planck. ( Memento of the original from June 29, 2011 in the Internet Archive ) Info: The  archive link was automatically inserted and not yet checked. Please check the original and archive link according to the instructions and then remove this notice. At: spiegel.de. April 24, 2008. Retrieved April 23, 2018.
4. Max Planck Society: The dispute about the "r": What was Max Planck really called? Press release of April 24, 2008, accessed on February 21, 2013.
5. a b Hermann: Planck. 6th edition. Reinbek near Hamburg 1995, p. 7.
6. ^ Hellmann: Johann Julius Wilhelm von Planck †. Obituary in the Deutsche Juristen-Zeitung, vol. 5 (1900), p. 409.
7. ^ Hoffmann: Max Planck. Munich 2008, p. 8 f.
8. ^ Hartmann: Max Planck as a man and a thinker. 3rd, revised edition. Basel 1953, p. 29 ff.
9. ^ Hartmann: Max Planck as a man and a thinker. 3rd, revised edition, Basel 1953, p. 39 f.
10. After: Fischer: Der Physiker. Munich 2007, pp. 31, 34.
11. ^ Max Planck: Lectures and Memories. 7th edition. Darmstadt 1969, p. 1; after: Hermann: Planck. 6th edition. Reinbek near Hamburg 1995, p. 7.
12. ^ Hoffmann: Max Planck. Munich 2008, p. 10.
13. ^ Hoffmann: Max Planck. Munich 2008, p. 13.
14. ^ Planck: Letter to Josef Strasser. December 14, 1930, after: Hermann: Planck. 6th edition, Reinbek bei Hamburg 1995, p. 11.
15. a b Hartmann: Max Planck as a man and a thinker. 3rd revised edition, Basel 1953, p. 12 ff.
16. ^ Association of Alter SVer (VASV): Address book. Membership directory of all old men. As of October 1, 1937. Hanover 1937, p. 172.
17. ^ Fischer: The physicist. Munich 2007, p. 40.
18. ^ Hermann: Planck. 6th edition, Reinbek bei Hamburg 1995, p. 11 ff.
19. Planck: Scientific autobiography. after: Fischer: The physicist. Munich 2007, p. 52 f.
20. ^ Hermann: Planck. 6th edition, Reinbek bei Hamburg 2005, p. 13 f.
21. Planck: About the second law of mechanical heat theory. Ackermann, Munich 1879.
22. ^ Hoffmann: Max Planck. Munich 2008, p. 14 ff.
23. a b Hoffmann: Max Planck. Munich 2008, p. 18 ff.
24. a b Hermann: Planck. 6th edition, Reinbek bei Hamburg 1995, p. 17 ff.
25. a b Hermann: Planck. 6th edition, Reinbek bei Hamburg 1995, p. 19 f.
26. ^ Hoffmann: Max Planck. Munich 2008, p. 21 f.
27. Planck: The principle of the conservation of energy. Leipzig 1887, after: Hoffmann: Max Planck. Munich 2008, p. 32.
28. ^ Hartmann: Max Planck as a man and a thinker. 3rd revised edition, Basel 1953, p. 16.
29. ^ Heilbron: The Dilemmas of an Upright Man. Berkeley 1986, p. 12.
30. ^ Hoffmann: Max Planck. Munich 2008, p. 36.
31. ^ Hoffmann: Max Planck. Munich 2008, p. 39 f.
32. ^ Hoffmann: Max Planck. Munich 2008, p. 41 f. and p. 69.
33. a b Hermann: Planck. 6th edition, Reinbek bei Hamburg 1995, p. 23 f.
34. ^ Hermann: Planck. 6th edition, Reinbek bei Hamburg 1995, p. 37 ff.
35. Hoffmann: Max Planck. Munich 2008, p. 49 ff.
36. ^ Hermann: Planck. 6th edition, Reinbek bei Hamburg 1995, p. 40 ff.
37. a b Hermann: Planck. 6th edition, Reinbek bei Hamburg 1995, p. 45 ff.
38. a b Hoffmann: Max Planck. Munich 2008, p. 63 ff.
39. a b Hermann: Planck. 6th edition, Reinbek bei Hamburg 1995, p. 48 f.
40. ^ Hoffmann: Max Planck. Munich 2008, p. 67 f.
41. a b c d Hermann: Planck. 6th edition, Reinbek bei Hamburg 1995, p. 50 ff.
42. ^ Hoffmann: Max Planck. Munich 2008, p. 70 f.
43. Planck: Physical treatises and lectures. Braunschweig 1948, Volume 3, p. 77 based on: Hoffmann: Max Planck. Munich 2008, p. 72.
44. a b Hoffmann: Max Planck. Munich 2008, p. 71 ff.
45. ^ Letter from Albert Einstein to Hendrik A. Lorentz dated August 2, 1915, in: Robert Schulmann u. a. (Ed.) 1998: The Collected Papers of Albert Einstein. Volume 8, Princeton University Press, ISBN 978-0-691-04849-9 .
46. Walther Jaenicke: 100 Years of the Bunsen Society, 1894–1994. Springer Verlag, 1994, ISBN 978-3-7985-0979-5 .
47. Hartmut Kaelbe and a .: Europe and the Europeans: Sources and essays on modern European history. Franz Steiner Verlag, 2005, ISBN 978-3-515-08691-2 .
48. Agnes von Zahn-Harnack: Adolf von Harnack. Walter de Gruyter, 1950, ISBN 978-3-11-003219-2 .
49. a b Kurt Nowak, Otto Gerhard Oexle (ed.): Adolf Von Harnack: Theologe, Historiker, Wissenschaftspolitiker (=  publications of the Max Planck Institute for History. Volume 161). Vandenhoeck & Ruprecht, Göttingen 2001, ISBN 978-3-525-35477-3 .
50. ^ Heilbron: The Dilemmas of an Upright Man. Berkeley 1986, p. 72.
51. ^ Hermann: Planck. 6th edition, Reinbek bei Hamburg 1995, p. 55 f.
52. ^ Heilbron: The Dilemmas of an Upright Man. Berkeley 1986, pp. 83 f.
53. ^ Hermann: Planck. 6th edition, Reinbek bei Hamburg 1995, p. 59 f.
54. ^ Margit Szöllösi-Janze: Fritz Haber, 1868–1934: A biography. CH Beck, 1998, ISBN 978-3-406-43548-5 .
55. Quoted from Ernst Klee : Das Personenlexikon zum Third Reich. Frankfurt 2003, p. 463, ISBN 3-10-039309-0 .
56. ^ A b Astrid von Pufendorf 2006: How Hitler killed Planck.
57. Wolfgang Ribbe (Ed.) 1987: Berlinische Lebensbilder. 1. Scientist. Volume 60 of individual publications by the Historical Commission in Berlin at the Friedrich Meinecke Institute of the Free University of Berlin. Colloquium Verlag, ISBN 978-3-7678-0697-9 .
58. ^ Max Planck: Lectures and Memories. Pp. 331-332.
59. Erich Dinkler: Planck, Max. In: The religion in history and present . 3rd ed., Volume V, columns 404-405.
60. ^ Hoffmann: Max Planck. Munich 2008, p. 27.
61. a b Hartmann: Max Planck as a man and a thinker. 3rd revised edition, Basel 1953, p. 156 f.
62. a b c Hoffmann: Max Planck. Munich 2008, p. 29.
63. ^ Hoffmann: Max Planck. Munich 2008, p. 31 f.
64. Armin Hermann: Early history of the quantum theory. Mosbach 1969, p. 31 based on: Hoffmann: Max Planck. Munich 2008, p. 61.
65. ^ Hermann: Planck. 6th edition, Reinbek bei Hamburg 1995, p. 35.
66. Max Planck: The natural mood in modern vocal music . In: Quarterly magazine for musicology . tape 9 , no. 4 . Breitkopf & Härtel, Berlin October 1893, p. 418-440 ( DigiZeitschriften ).
67. Information from the Nobel Foundation on the award ceremony for Max Planck in 1918 (English). - Les Prix Nobel, documents submitted.
68. ^ Fellows Directory. Biographical Index: Former RSE Fellows 1783–2002. (PDF file) Royal Society of Edinburgh, accessed March 30, 2020 .
69. Planetary Names: Crater, craters: Planck on Moon. In: Gazetteer of Planetary Nomenclature. Retrieved August 30, 2018 .
70. Planetary Names: Vallis, valles: Vallis Planck on Moon. In: Gazetteer of Planetary Nomenclature. Retrieved August 30, 2018 .
71. ^ Initiative Max Planck Museum Kiel and other honors from Max Planck in Kiel
 This version was added to the list of articles worth reading on July 15, 2005 .